This invention relates to aqua-ammonia absorption cooling and/or heating systems utilizing ammonia refrigerant and aqueous absorbents. Improvements in the efficiencies of such systems include the use of generator/absorber heat exchange cycles utilizing rich and weak absorption working fluids and/or by separate heat exchange loops referred to as GAX cycles. Descriptions of such systems are found in U.S. Pat. Nos. 4,311,019, 5,024,063, 5,271,235, 5,367,884, Re. 36,684 and R. J. Modahl and F. C. Hayes, xe2x80x9cEvaluation of Commercial Advanced Absorption Heat Pump Bread Board,xe2x80x9d The Trane Company, pp. 117-125, 1988. Additional improvements are described in co-pending U.S. patent application Ser. No. 479,277, filed Jan. 5, 2000 Ser. No. 632,037, filed Aug. 3, 2000 Ser. No. 632,054, filed Aug. 3, 2000; and Ser. No. 709,875, filed Nov. 10, 2000. The description of the aforesaid patents and applications are incorporated herein by reference.
The present invention is directed to further improvement in efficiency of aqua-ammonia absorption systems by utilizing a thermostatic expansion valve (TXV) for controlling refrigerant flow into the evaporator. The use of TXVs with aqua-ammonia cycles is intended to provide correct fill of the evaporator by controlling superheat of vapor exiting the evaporator although such valves must be modified to work properly with a solution of water and ammonia refrigerant. Some aqua-ammonia equipment exhibits greater change in capacity as ambient temperature changes compared to vapor compression appliances. The relatively slow start-up of aqua-ammonia equipment results in a low-capacity operation over longer periods of time. Although conventional TXVs may provide good control of evaporator fill over a narrow range of refrigerant flow rates, it has been found that the pulsed-operation control valve described in U.S. Pat. No. 5,675,982 provides enhanced low-capacity control capability, e.g. large turn-down capability. Excellent control with large turn-down is not only beneficial for startup, but is also useful in systems with multiple or variable speed burners. TXVs are controlled by a temperature-sensing bulb located in the superheat region of the evaporator tube. The bulb is charged with a composition that produces or exerts a pressure proportional to the temperature sensed by the bulb. The bulb pressure change is directed to a movable member such as a diaphragm or bellows in the valve for opening and closing the valve in response to pressure differential between the surfaces of the diaphragm. Moreover, the performance of TXVs in refrigerant flow in aqua-ammonia absorption systems is dependent on using a bulb charge composition that functions properly with other valve design parameters.
In the system described herein a TXV cooperates with refrigerant piping for controlling the flow of refrigerant to the evaporator in an aqua-ammonia heating and/or cooling system. In one embodiment, a pulsed operation TXV is used in combination with a temperature-sensing bulb in thermal contact with the evaporator, preferably near the evaporator outlet, for controlling refrigerant flow to the evaporator, with the bulb located above the valve diaphragm.
In another embodiment of the invention, a bulb charge composition for operating a TXV for controlling superheat of vapor in an evaporator in an aqua-ammonia heating and/or cooling system comprises a mixture of water and ammonia, preferably between about 70% and 99% ammonia, and more preferably between about 80% and 90% ammonia, by weight.
In another embodiment, a bulb charge composition for operating a TXV for controlling superheat of vapor in an evaporator in an aqua-ammonia heating and/or cooling system comprises a mixture of butane and propane, preferably a major amount of propane and minor amount of butane, and more preferably between about 10% and about 40% butane and between about 90% and about 60% propane, by weight.